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Illumination Design Gets Faster, Easier

Photonics SpectraApr 2003In the development of illumination systems, many factors complicate the designer's job. Design software not only is making it easier, but also is shortening the design cycle.

Bruce Irving

From cell phones to automotive dashboards, illuminated displays have become commonplace.
Historically, they were designed with rudimentary software tools, and, consequently,
many prototype iterations were required. But in the past decade, the explosive growth
in the market has fueled the development of software packages specifically constructed
for illumination system design.

These programs rely on ray tracing. They propagate
light rays through-out the system and take into account such issues as refraction,
reflection and scattering at each interface and medium. Because conventional optical
design programs also use ray tracing, there may be confusion between these two types
of software. However, optical and illumination system design programs are not interchangeable.
They use different approaches in the way they model systems, perform ray tracing
and provide output. There are also important differences in the user interface for
the two types of software, reflecting the different tasks required.

Optical vs. illumination design

In virtually all optical design programs, the
object is modeled as a point source, or a series of point sources at positions on
and off an optical axis. The program creates a predetermined set of rays —
a ray fan — emanating from each of these points.

In contrast, advanced illumination
design programs model light sources as points, surfaces or volumes, or any combination
of these elements. The spatial and angular distribution of a source’s output,
as well as its spectral characteristics, are specified. Moreover, structural elements
of the source — the glass envelope of a bulb or the heat sink in an LED, for
example — may also be included. The generation of a Monte Carlo ray set is
based on this source model. This is a random, rather than predetermined, set of
rays that emanates from throughout the source in accordance with its distribution
characteristics.

In typical optical system design software,
nonoptical surfaces such as edges and bevels, and mechanical constructs such as
mounts, usually are either ignored or only partially represented. Ray tracing involves
propagating a ray from one optical surface to the next in sequence along the optical
axis; when a ray misses a surface, is directed out of the system or hits a nonoptical
surface, such as a component edge, it may be ignored or may generate an error.

However, illumination design packages
use models that are reminiscent of mechanical computer-aided design (CAD) software.
Each component is represented as a complete, three-dimensional object, with sides
and edges. Other elements, such as mechanical mounts, housings, mounting hardware
and circuit boards, also can be included in the system. The optical properties of
all these surfaces are specified. In some programs, surfaces may be defined as nonuniform,
containing arrays of bumps, grooves or paint spots that may vary in density with
position. Nor are there fixed limits on the number of inputs and outputs. For example,
multiple light sources might be used, or a single source might illuminate several
points in a display.

The procedures and user interface employed
to create this type of 3-D model are similar to those used in mechanical CAD software.
Usually a toolbox of common shapes is provided, and Boolean operations (unions,
intersections) enable the user to quickly create complex elements. A visual, interactive
interface is important in this process because even many commonly encountered shapes
are quite difficult to describe mathematically. Once it has been created, object
orientation and position in 3-D space are controlled without reference to an optical
axis.

It is also important to note that ray
tracing in illumination software is nonsequential; there is no predetermined order
in which a ray is expected to encounter surfaces. Thus, a ray may exit the source,
be reflected toward the source by a lens-retaining ring, then be reflected toward
the first lens by the source envelope, and so on. Just about any interaction between
light and any of the modeled elements is possible.

With most conventional optical systems,
specifying system performance using parameters such as spot diagrams, point spread
functions, aberration curves and modulation transfer functions is straightforward.
Commercial software packages generate and plot these functions in a readily accessible
format, and high-performance programs provide design optimization algorithms. This
is possible in large part because it’s relatively simple to define what constitutes
good performance for most imaging systems — for example, minimum spot size
or wavefront distortion — and because there are a limited number of system
parameters to be varied.

Specifying and optimizing illumination
system performance is not as easy. Illumination design programs typically provide
angular and spatial distribution plots of light energy, usually calibrated in physical
units such as lumens, lux and watts per square meter. The primary purpose of these
plots is to deliver an accurate representation of what the human eye would see when
viewing an actual system — in other words, brightness or change in brightness
as a function of viewing angle. This process must be interactive so that the designer
can rapidly make changes to the system and see the results immediately.

Software iterations are required because
optimization is a more open-ended process in illumination system design. There are
many reasons for this; for instance, performance metrics are typically application-specific,
and many more system parameters may be varied to achieve the desired output. Currently,
no commercial illumination design programs offer a general optimization capability,
but this is an active area of research and the future looks promising.

With these factors in mind, the best
way to understand the usefulness and potential of illumination design software is
to examine some typical applications.

Primary design objectives for LCD backlights
are to couple the maximum amount of light into the specified viewing angle, to achieve
the desired falloff in brightness as a function of viewing angle, to deliver uniform
illumination over the entire display and, for full-color displays, to achieve uniform
color. The latter can be a challenge with LED-based systems because full color is
usually achieved by combining light from red, green and blue sources. Moreover,
because LCDs use polarized light, it is helpful if the design software adequately
models polarization effects.

LED backlights typically employ light
pipes to combine the output of multiple sources and to transform it into a uniform
line. The output from a light pipe is directed into the edge of a diffuser that
sits underneath the display and redirects light toward the user. However, the flux
in the diffuser decreases with distance from the input light pipe, so the diffuser
response must vary accordingly.

To accomplish this, a matrix of paint
spots with variable spacing is placed on the back of the diffuser. However, producing
this paint spot pattern presents some production difficulties and doesn’t
deliver optimum efficiency. A better solution would be to mold a series of bumps
directly into the diffuser. Until recently this approach was more difficult to model,
but advances in illumination design software should make it much more attractive
to manufacturers (Figure 1).

Figure 1. In large LCDs, light from a cold-cathode fluorescent lamp (CFL) is introduced into the edge of the display. An array of bumps in a diffuser placed behind the display evenly channels illumination toward the viewer’s eyes.Data projection

Another key application is in digital data projectors.
Over the past decade, these have steadily become smaller, brighter and less expensive,
while increasing in output resolution. Improvements in illumination system design
have been a key component in this evolution.

The main design goals in projector
illumination are to deliver high source-collection efficiency and high output uniformity
in a compact package. These goals necessitate a different approach from traditional
projection optics, such as overhead or movie projectors, which typically utilize
a condenser system that trades off collection efficiency against uniformity. To
achieve both, digital data projectors usually rely on a pair of lamps with either
a mixing rod or lenslet arrays.

A mixing rod is a long light pipe used
to homogenize source output. Relay optics transfer its output to a spatial light
modulator. Numerous mixing rod design forms have been investigated. The goal is
to achieve maximum uniformity in minimum length. In terms of software capabilities,
the ability to accurately model light pipe performance and investigate the effects
of real-world tolerancing considerations, such as misalignments, is critical to
meeting these goals. Today, the most commonly used solution is a rod with a rectangular
cross section that matches the shape of the modulator. Usually, a length-to-width
ratio of about 10:1 is required to achieve the necessary uniformity; shrinking this
length and reducing overall system cost are the present focus of most design efforts.

Lenslet arrays are rectangular arrangements
of small lenses. These can provide a package-size advantage over light pipes, although
they generally have a higher initial tooling cost (Figure 2).

Figure 2. Pairs of lenslet arrays can be used to homogenize the source output in digital
data projectors. This approach results in a compact package.
Some projector designs utilize polarized
light and dichroic filters. Software must therefore be able to model these components
and must include effects such as the angular dependency of dichroic coatings.

In the automotive world, backlit controls,
gauges and displays are the prevailing standard. A major consideration is the total
cost of the sources because a typical car interior has 50 or more light sources.
Anything that can reduce this count has a tremendous cost impact for the manufacturer.

A typical automotive illumination system
feeds an incandescent or LED source into several light pipes. For example, a single
source might light an entire row of buttons on a sound system or a climate control
center (Figure 3). Sometimes the controls or indicators being lighted may have variable
position; e.g., speedometer needles or volume control knobs. Here, total internal
reflection, rather than paint spots or bumps, redirects the light.

Figure 3. In these front and back views of automotive radio knobs, a single source evenly
illuminates several graphics.
Because of the tremendous diversity
of shapes and layouts encountered, there is no single design form for these tasks,
complicating the designer’s job. Furthermore, there may be significant manufacturing
variations in the parts, such as surface smoothness in light pipes. Consequently,
it is still necessary to construct prototypes, and a major goal of the design process
is to minimize the number required.

All of this makes the design of automotive
illumination systems a very interactive process. After creating an initial form,
the designer must be able to rapidly alter and view the results to investigate various
scenarios. It is also critical to model surrounding mechanical mounts in this process
because they constrain light pipe geometry and can alter the system output.

In summary, illumination design is
now performed with software tools specifically developed for this task. These tools
must provide high-quality visualization, interactive ray tracing to enable real-time
design decisions and connectivity with other tools such as CAD programs. They must
also help to shorten the overall design cycle. This software has already played
a key role in the development of a number of consumer products. As illumination
design software becomes even more powerful and easier to use, it should deliver
more cost-effective solutions and find greater applicability.

Meet the author

Bruce Irving is manager of international distribution for Optical Research Associates in?Westborough, Mass.